Defining Net-Zero

When it comes to residential housing, the process of selecting just the right house that fulfills your needs at an affordable price, can be quite difficult. Sustainable housing options often add another layer of unwanted, and frankly unneeded, complexity and cost to an already laborious process. Despite the relative opacity of the green housing market, residential green construction has skyrocketed from 2 percent of new homes in 2005 to 17 percent of new homes in 2011, according to a McGraw Hill Market survey report. The same report found that, despite the dismal housing market, 61 percent of customers are willing to pay more for homes that are energy-efficient and have other green features. Driving this unexpected growth is a type of ultra-efficient
, sustainable housing, known as net-zero housing, which has gained traction throughout the United States, especially progressive and sunny places like Austin, Texas.

Due to the popularity of reducing residential energy consumption, the term “zero energy building” is widely used. However, a common criterion for defining and evaluating whether a house is net-zero is not widely available. The Building America Program defines a net-zero energy building (ZEB) generally as, “a residential or commercial building with greatly reduced energy needs through efficiency gains such that the balance of energy needs can be supplied with renewable technologies.” Of course this could mean anything from a shed made of biodegradable straw bales to a suburban tract home plastered with photovoltaics – the first is not particularly comfortable, and the latter is both expensive and not particularly sustainable or attractive to look at. Listed below are the four defined categories of net-zero houses as determined by the U.S. Department of Energy. Despite the superficial similarity of the terms, each definition has advantages and disadvantages that translate into substantial performance results.

4 Different Definitions (Site, Source, Cost, and Emission)

Net Zero Site Energy Building

In this type of ZEB, the amount of energy provided by on-site renewable energy sources, such as solar panels, is equal to the amount of energy used by the building. In the United States, “zero net energy building” generally refers to this type of building. Builders generally prefer this type of ZEB since it is both easy to implement and easy to measure in terms of performance. However, for a house with significant gas use, this definition would be untenable since the building would need to produce markedly more electricity since electricity is more than three times more valuable at the source. High electricity consumption could contribute to little or no appreciable energy cost savings compared to a non-net-zero home if peak energy use is not managed.

Net Zero Source Energy Building

This type of ZEB produces as much energy as it uses as measured at the source making it a better measure of the actual energy balance. However, it suffers from the same lack of discernible energy cost savings if peak energy usage is not managed.

Net Zero Energy Cost Building

A cost ZEB receives as much financial credit for exported energy as it is charged on the utility bills. Unfortunately, Net Zero Energy Cost Buildings are the most sensitive to local energy costs and utility rate structures. This means that while your cost ZEB house may be considered “net-zero” today, tomorrow it may not be based solely on fluctuating utility prices. This definition is difficult to achieve in net-zero buildings and difficult to implement since net metering capabilities are not consistently available.

Net Zero Energy Emissions Building

An emissions-based ZEB is a good approach to sustainable building as it takes into account and offsets carbon emissions for its entire lifecycle – from construction to actual inhabitation.

While many may claim to offer “net-zero” homes, the actual definition can be a bit murky depending on how and where the energy flow is measured. Many houses on the market today would more appropriately be labeled “low-energy” or fall under a broad class of somewhat eco-friendly buildings that look good on paper but don’t perform quite so well. For a quick rundown on what really makes a good net-zero home, see the resource “The 5 Ingredients of Net-zero Homes”.